Ion Mobility Separation Device

ABSTRACT

An ion mobility separator and a method of separating ions according to their ion mobility are disclosed. An RF ion guide is provided having a plurality of electrodes that are arranged to form an ion guiding path that extends in a closed loop. RF voltages are supplied to at least some of the electrodes in order to confine ions within said ion guiding path. A DC voltage gradient is maintained along at least a portion of a longitudinal axis of the ion guide, wherein the voltage gradient urges ions to undergo one or more cycles around the ion guide and thus causes the ions to separate according to their ion mobility as the ions pass along the ion guide. The closed loop ion guide enables the resolution of the ion mobility separator to be increased without necessitating a large device, since the drift length through the device can be increased by causing the ions to undergo multiple cycles around the device.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.14/367,613, which is the National Stage of International Application No.PCT/GB2012/053254, filed 21 Dec. 2012, which claims priority from andthe benefit of U.S. Provisional Patent Application Ser. No. 61/580,547filed 27 Dec. 2011 and United Kingdom Patent Application No. 1122251.1filed on 23 Dec. 2011. The entire contents of these applications areincorporated herein by reference.

BACKGROUND OF THE PRESENT INVENTION

It is known to apply a uniform electric field across a drift region ofan ion mobility spectrometer (IMS) in order to separate ions accordingto their ion mobilities. It is desirable to provide such devices withrelatively high resolution. It is possible to increase the resolution ofsuch a device by increasing the electric field strength in the driftregion. However, this will ultimately result in electrical breakdown inthe drift gas. In order to increase the resolution of the device it istherefore conventionally considered necessary to increase the length ofthe drift region, whilst maintaining the electric field strength.However, this leads to a relatively long IMS device and the use of alarger potential difference in order to maintain the same electric fieldstrength over the longer drift region. This necessitates the use of highabsolute voltages, which may result in hazardous electrical dischargesto the surrounding areas.

It is therefore desired to provide an improved ion mobility separatorand an improved method of separating ions according to their ionmobility.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a method of separating ions according totheir ion mobility comprising:

providing an RF ion guide having a plurality of electrodes arranged toform an ion guiding path that extends in a closed loop;

supplying RF voltages to at least some of said electrodes in order toconfine ions within said ion guiding path; and

maintaining a DC voltage gradient along at least a portion of alongitudinal axis of said ion guide, wherein said voltage gradient urgesions to undergo one or more cycles around said ion guide and thus causesthe ions to separate according to their ion mobility as they pass alongthe ion guide.

Conventionally it has been necessary to employ a relatively long driftregion in order to obtain the desired resolution of ion mobilityseparation. The closed loop ion guide of the present invention enablesthe resolution of the ion mobility separation to be increased withoutnecessitating a large device, since the drift length through the devicecan be increased by causing the ions to undergo multiple cycles aroundthe device.

The ion guide preferably comprises an ion entry/exit region configuredfor introducing ions into the ion guide in one mode and for extractingions from the ion guide in another mode, wherein the ion entry/exitregion is at a fixed location on the ion guide.

Preferably, the electrodes of the ion guide are axially spaced along thelongitudinal axis of the ion guide and different DC voltages are appliedto different ones of said axially spaced electrodes so as to form saidDC voltage gradient.

The DC voltage gradient region described herein is preferably definedover a length of the ion guide extending from a first electrode at arelatively high potential to a second electrode at a relatively lowpotential. Progressively smaller DC potentials are preferably applied toelectrodes between the first and second electrodes in a direction fromthe first electrode to the second electrode so as to form said voltagegradient. This differs from arrangements wherein a voltage step orbarrier is conveyed along a series of electrodes. It is also preferredin the present invention that the ions separate out according to theirion mobility within the DC voltage gradient region.

It is preferred that a substantially uniform DC voltage gradient isarranged along the DC voltage gradient region. In embodiments whereinthe ion guide is formed from axially spaced electrodes, this may beachieved by providing relatively small potential differences betweenpairs of adjacent electrodes in the DC voltage gradient region such thatthe DC potential decreases progressively and gradually along thisregion.

Preferably, as time progresses the portion of the ion guide along whichthe DC voltage gradient is maintained is moved along the ion guide. Forexample, the DC voltage gradient may chase the ions along the ion guidesuch that the ions remain within the DC voltage gradient, even as theypass around the ion guide. This may be advantageous in that a relativelysmall potential difference can be used to set up a relatively largeelectric field strength over a relatively small region, and this regioncan then be moved along the ion guide such that the ions remain in therelatively high strength electric field as they travel around the ionguide. The ions can therefore be separated in an electric field regionof relatively high strength but without having to apply such a highelectric field strength along the whole ion guide at any given time,which would require a larger potential difference to be applied across alonger length.

Alternatively, or additionally, the voltage gradient may be moved suchthat the ions can remain on the same DC voltage gradient as they passaround the ion guide multiple times. If the DC voltage gradient remainedin a fixed location and the ions passed around the ion guide multipletimes then at some point the ions would have to make a transition fromthe low potential end of the voltage gradient back to the high potentialend of the voltage gradient. It is undesirable that the ions travelacross such a voltage discontinuity since it is desired that the ionsonly travel through a substantially uniform and continuous DC voltagegradient during their ion mobility separation.

The DC voltage gradient preferably moves around the ion guide at a ratesuch that at least some of said ions continually remain within the DCvoltage gradient region as they travel around the ion guide andpreferably until such ions are extracted from the ion guide at an exitregion of the ion guide. The ions may remain within the DC voltagegradient as they travel only a single cycle around the closed loop ionguide. Alternatively, the ions may remain within the DC voltage gradientas they repeatedly travel multiple cycles around the closed loop ionguide.

The DC voltage gradient may be moved around the ion guide at a rate suchthat undesired ions having an ion mobility above a first threshold valueand/or below a second threshold value do not continuously remain withinthe voltage gradient region as the region is moved around the ion guide.The rate at which the voltage gradient is moved around the ion guide maycause undesired ions having an ion mobility above the first thresholdvalue to exit the low potential end of the voltage gradient regionand/or may cause undesired ions having an ion mobility below the secondthreshold value to exit the high potential end of the voltage gradient.The undesired ions that do not continuously remain within the DC voltagegradient region may not be extracted from the ion guide at an exitregion of the ion guide.

RF voltages are applied to the electrodes so as to confine ions withinthe ion guiding path along the DC voltage gradient region. Such RFvoltages may not be applied to at least some of the electrodes at one ormore regions of the ion guide outside of the DC voltage gradient regionsuch that ions are not confined within said one or more regions of theion guide and are lost from the ion guide at these regions. This is anefficient method of removing undesired ions that do not remain on the DCvoltage gradient.

The rate at which the DC voltage gradient moves around the ion guide maybe synchronised with the rate at which ions of interest are urged aroundthe ion guide by the voltage gradient such that the ions of interestreach an exit region of the ion guide at a time when the minimumpotential of the voltage gradient is arranged at the exit region of theion guide. This is advantageous as it may be desirable to maintain arelatively high potential region of the voltage gradient at the exitregion at one time during the ion mobility separation process, but forthe potential at the exit region to be low or zero at the time that theions exit from the exit region. For example, if the exit region andentrance region are collocated at the same region, then it may bedesirable that substantially the maximum potential of the voltagegradient is arranged at the entrance region at a time when the ions arein the entrance region so as to urge the ions away from the entranceregion and around the ion guide, but that by the time the ions havepassed around the ions guide and reached the exit region the minimumpotential of the voltage gradient is arranged at the exit region, e.g.to enable efficient extraction of the ions from the exit region.

The DC voltages are preferably only applied to some of the electrodes ofthe ion guide such that the DC voltage gradient is arranged along only aportion of the length of the ion guide at any given time. At any giventime the DC voltage gradient may be arranged over only a percentage ofthe length of the ion guide selected from: <5%; <10%; <20%; <30%; <40%;<50%; <60%; <70%; <80%; or <90%. Additionally, or alternatively, at anygiven time the DC voltage gradient may be arranged over only apercentage of the length of the ion guide selectedfrom: >5%; >10%; >20%; >30%; >40%; >50%; >60%; >70%; >80%; or >90%. Anypermutation of ranges from the above two lists may be combined.

Alternatively, the DC voltage gradient may be arranged oversubstantially the whole length of the ion guiding region at any giventime.

The electrodes are preferably configured to confine ions in directionsperpendicular to the longitudinal axis of the ion guide when said RFvoltages are applied.

The electrodes are preferably apertured electrodes that are aligned suchthat the ions are guided through the apertures of the electrodes as theytravel along the ion guiding path. Preferably, the apertures in theelectrodes are slotted apertures. in this embodiment, the electrodes maybe arranged such that at least a portion of the ion guiding path iscurved and so has a radius of curvature, wherein each slot has itsminimum dimension substantially parallel with said radius and itsmaximum dimension substantially perpendicular to said radius.

The electrodes are preferably arranged such that the closed loop ionguiding path is substantially circular or oval. However, any otherclosed loop geometry may be used.

A drift gas is preferably arranged in said ion guide such that ionsseparate according to their mobility through the drift gas as they areurged along the ion guide.

Ions may be introduced into or ejected out of the closed loop ion guidethrough a side of the ion guide. Alternatively, or additionally ions maybe introduced into the closed loop ion guide through the top or bottomof the ion guide; and/or ions may be ejected out of the closed loop ionguide through the top or bottom of the ion guide.

An array of electrodes may be provided to urge ions into an entry regionof the ion guide; and/or an array of electrodes may be provided in anexit region of the ion guide to urge ions out of the ion guide.

The ion guide may comprise an exit region and ions may be ejected out ofion guide at the exit region as the ions travel around the ion guide byapplying a voltage pulse to one or more electrodes of the ion guide, thetiming of the voltage pulse being selected so as eject ions of aselected ion mobility as they pass through the exit region.

Although the electrodes forming the ion guiding path have been describedhereinabove as being apertured electrodes, it is also contemplated thatother geometries of electrodes may be used to guide ions around thedevice. For example, the ion guide may be segmented in the longitudinaldirection into a plurality of segments and each segment may comprise aplurality of electrodes arranged and configured for confining andguiding the ions. Each segment preferably comprises a top electrode, abottom electrode and a plurality of side electrodes arrangedtherebetween so as to define a space between the top, bottom and sideelectrodes through which ions are guided. RF potentials are preferablyapplied to the side electrodes so as to confine ions in said space inthe direction between the side electrodes. DC potentials are preferablyapplied to the top and/or bottom electrodes so as to confine ions insaid space in the direction between the top and bottom electrodes. Lesspreferably, RF potentials are applied to the top and/or bottomelectrodes so as to confine ions in said space in the direction betweenthese electrodes.

Each segment may comprise a plurality of layers of side electrodesarranged between the top and bottom electrodes. Each layer preferablycomprises two laterally spaced apart electrodes, which define a spacetherebetween for guiding ions. The side electrodes are preferablystacked in columns so as to define a space between the columns of sideelectrodes, and between the top and bottom electrodes. The top, bottomand side electrodes are preferably substantially planar and extendaround the longitudinal direction of the drift cell so as to form asegment of the drift cell. The electrodes may be formed from printedcircuit boards.

Ions may be radially confined within the space between the sideelectrodes, top electrode and bottom electrode by applying RF potentialsto the side electrodes. The same phase of an RF voltage source ispreferably applied to the two side electrodes in each layer. Differentphases of the RF voltage source are preferably applied to the sideelectrodes in adjacent layers. The side electrodes in any given layerare preferably supplied with an opposite RF voltage phase to the sideelectrodes in the adjacent layers. By applying RF potentials to the sideelectrodes, the ions are laterally confined within the space between theside electrodes. RF potentials may also be applied to the top and bottomelectrodes so as to confine ions within the space in the verticaldirection. However, it is preferred that only DC potentials are appliedto the top and bottom electrodes so as to confine the ions in thevertical direction.

A DC voltage gradient is preferably applied to at least some of theelectrodes so as to provide an axial electric field that urges ions todrift through the drift gas and around the drift cell. The DC voltagegradient may be formed by supplying different DC voltages to theelectrodes of different segments of the drift cell. Different DCvoltages may be supplied to the top and/or bottom electrode in differentsegments so as to form the voltage gradient. Additionally, oralternatively, different DC voltages may be supplied to the sideelectrodes of different segments so as to form the voltage gradient. Forexample, progressively smaller DC voltages may be applied to theelectrodes of the different segments around the drift cell so as tocreate a voltage gradient that drives the ions along the drift length.

The present invention also provides a method of mass spectrometrycomprising separating ions according to any one of the methods describedabove.

The present invention also provides an ion mobility separatorcomprising:

an RF ion guide having a plurality of electrodes arranged to form an ionguiding path that extends in a closed loop;

an RF voltage supply for supplying RF voltages to said electrodes forconfining ions within said ion guiding path; and

a DC voltage supply arranged and adapted to maintain a DC voltagegradient along at least a portion of a longitudinal axis of said ionguide, wherein in use said voltage gradient urges ions to undergo one ormore cycles around said ion guide and thus to cause the ions to separateaccording to their ion mobility as they pass along the ion guide.

The ion mobility separator may be arranged and configured to perform anyone of the method of separating ions that has been described above.

The present invention also provides a mass spectrometer comprising anion mobility spectrometer as described above.

According to an embodiment the mass spectrometer may further comprise:

(a) an ion source selected from the group consisting of: (i) anElectrospray ionisation (“ESI”) ion source; (ii) an Atmospheric PressurePhoto Ionisation (“APPI”) ion source; (iii) an Atmospheric PressureChemical Ionisation (“APCI”) ion source; (iv) a Matrix Assisted LaserDesorption Ionisation (“MALDI”) ion source; (v) a Laser DesorptionIonisation (“LDI”) ion source; (vi) an Atmospheric Pressure Ionisation(“API”) ion source; (vii) a Desorption Ionisation on Silicon (“DIOS”)ion source; (viii) an Electron Impact (“EI”) ion source; (ix) a ChemicalIonisation (“CI”) ion source; (x) a Field Ionisation (“FI”) ion source;(xi) a Field Desorption (“FD”) ion source; (xii) an Inductively CoupledPlasma (“ICP”) ion source; (xiii) a Fast Atom Bombardment (“FAB”) ionsource; (xiv) a Liquid Secondary Ion Mass Spectrometry (“LSIMS”) ionsource; (xv) a Desorption Electrospray Ionisation (“DESI”) ion source;(xvi) a Nickel-63 radioactive ion source; (xvii) an Atmospheric PressureMatrix Assisted Laser Desorption Ionisation ion source; (xviii) aThermospray ion source; (xix) an Atmospheric Sampling Glow DischargeIonisation (“ASGDI”) ion source; (xx) a Glow Discharge (“GD”) ionsource; and (xxi) an Impactor ion source; and/or

(b) one or more continuous or pulsed ion sources; and/or

(c) one or more additional ion guides; and/or

(d) one or more additional ion mobility separation devices and/or one ormore Field Asymmetric Ion Mobility Spectrometer devices; and/or

(e) one or more ion traps or one or more ion trapping regions; and/or

(f) one or more collision, fragmentation or reaction cells selected fromthe group consisting of: (i) a Collisional Induced Dissociation (“CID”)fragmentation device; (ii) a Surface Induced Dissociation (“SID”)fragmentation device; (iii) an Electron Transfer Dissociation (“ETD”)fragmentation device; (iv) an Electron Capture Dissociation (“ECD”)fragmentation device; (v) an Electron Collision or Impact Dissociationfragmentation device; (vi) a Photo Induced Dissociation (“PID”)fragmentation device; (vii) a Laser Induced Dissociation fragmentationdevice; (viii) an infrared radiation induced dissociation device; (ix)an ultraviolet radiation induced dissociation device; (x) anozzle-skimmer interface fragmentation device; (xi) an in-sourcefragmentation device; (xii) an in-source Collision Induced Dissociationfragmentation device; (xiii) a thermal or temperature sourcefragmentation device; (xiv) an electric field induced fragmentationdevice; (xv) a magnetic field induced fragmentation device; (xvi) anenzyme digestion or enzyme degradation fragmentation device; (xvii) anion-ion reaction fragmentation device; (xviii) an ion-molecule reactionfragmentation device; (xix) an ion-atom reaction fragmentation device;(xx) an ion-metastable ion reaction fragmentation device; (xxi) anion-metastable molecule reaction fragmentation device; (xxii) anion-metastable atom reaction fragmentation device; (xxiii) an ion-ionreaction device for reacting ions to form adduct or product ions; (xxiv)an ion-molecule reaction device for reacting ions to form adduct orproduct ions; (xxv) an ion-atom reaction device for reacting ions toform adduct or product ions; (xxvi) an ion-metastable ion reactiondevice for reacting ions to form adduct or product ions; (xxvii) anion-metastable molecule reaction device for reacting ions to form adductor product ions; (xxviii) an ion-metastable atom reaction device forreacting ions to form adduct or product ions; and (xxix) an ElectronIonisation Dissociation (“EID”) fragmentation device; and/or

(g) a mass analyser selected from the group consisting of: (i) aquadrupole mass analyser; (ii) a 2D or linear quadrupole mass analyser;(iii) a Paul or 3D quadrupole mass analyser; (iv) a Penning trap massanalyser; (v) an ion trap mass analyser; (vi) a magnetic sector massanalyser; (vii) Ion Cyclotron Resonance (“ICR”) mass analyser; (viii) aFourier Transform Ion Cyclotron Resonance (“FTICR”) mass analyser; (ix)an electrostatic or orbitrap mass analyser; (x) a Fourier Transformelectrostatic or orbitrap mass analyser; (xi) a Fourier Transform massanalyser; (xii) a Time of Flight mass analyser; (xiii) an orthogonalacceleration Time of Flight mass analyser; and (xiv) a linearacceleration Time of Flight mass analyser; and/or

(h) one or more energy analysers or electrostatic energy analysers;and/or

(i) one or more ion detectors; and/or

(j) one or more mass filters selected from the group consisting of: (i)a quadrupole mass filter; (ii) a 2D or linear quadrupole ion trap; (iii)a Paul or 3D quadrupole ion trap; (iv) a Penning ion trap; (v) an iontrap; (vi) a magnetic sector mass filter; (vii) a Time of Flight massfilter; and (viii) a Wein filter; and/or

(k) a device or ion gate for pulsing ions; and/or

(l) a device for converting a substantially continuous ion beam into apulsed ion beam.

The mass spectrometer may further comprise either:

(i) a C-trap and an orbitrap (RTM) mass analyser comprising an outerbarrel-like electrode and a coaxial inner spindle-like electrode,wherein in a first mode of operation ions are transmitted to the C-trapand are then injected into the orbitrap (RTM) mass analyser and whereinin a second mode of operation ions are transmitted to the C-trap andthen to a collision cell or Electron Transfer Dissociation devicewherein at least some ions are fragmented into fragment ions, andwherein the fragment ions are then transmitted to the C-trap beforebeing injected into the orbitrap (RTM) mass analyser; and/or

(ii) a stacked ring ion guide comprising a plurality of electrodes eachhaving an aperture through which ions are transmitted in use and whereinthe spacing of the electrodes increases along the length of the ionpath, and wherein the apertures in the electrodes in an upstream sectionof the ion guide have a first diameter and wherein the apertures in theelectrodes in a downstream section of the ion guide have a seconddiameter which is smaller than the first diameter, and wherein oppositephases of an AC or RF voltage are applied, in use, to successiveelectrodes.

According to an embodiment the mass spectrometer further comprises adevice arranged and adapted to supply an AC or RF voltage to theelectrodes. The AC or RF voltage preferably has an amplitude selectedfrom the group consisting of: (i) <50 V peak to peak; (ii) 50-100 V peakto peak; (iii) 100-150 V peak to peak; (iv) 150-200 V peak to peak; (v)200-250 V peak to peak; (vi) 250-300 V peak to peak; (vii) 300-350 Vpeak to peak; (viii) 350-400 V peak to peak; (ix) 400-450 V peak topeak; (x) 450-500 V peak to peak; and (xi) >500 V peak to peak.

The AC or RF voltage preferably has a frequency selected from the groupconsisting of: (i) <100 kHz; (ii) 100-200 kHz; (iii) 200-300 kHz; (iv)300-400 kHz; (v) 400-500 kHz; (vi) 0.5-1.0 MHz; (vii) 1.0-1.5 MHz;(viii) 1.5-2.0 MHz; (ix) 2.0-2.5 MHz; (x) 2.5-3.0 MHz; (xi) 3.0-3.5 MHz;(xii) 3.5-4.0 MHz; (xiii) 4.0-4.5 MHz; (xiv) 4.5-5.0 MHz; (xv) 5.0-5.5MHz; (xvi) 5.5-6.0 MHz; (xvii) 6.0-6.5 MHz; (xviii) 6.5-7.0 MHz; (xix)7.0-7.5 MHz; (xx) 7.5-8.0 MHz; (xxi) 8.0-8.5 MHz; (xxii) 8.5-9.0 MHz;(xxiii) 9.0-9.5 MHz; (xxiv) 9.5-10.0 MHz; and (xxv) >10.0 MHz.

The preferred embodiment provides the capability to undertake ionmobility separation around a closed loop system, wherein single ormultiple passes of the ions around the loop may be undertaken. As theion guide is formed as a closed loop the ions can pass around the loopmultiple times such that the closed loop ion guide provides a relativelylong drift region along which the ions can separate, whilst maintaininga relatively compact geometry.

Conventional ion mobility separator mass spectrometers generally employrelatively large ion mobility separators having relatively long, lineardrift regions for providing high resolution. These systems require theuse of high voltages to generate the required electric field across thelong drift region. Multi-pass IMS systems have been built but sufferfrom relatively low sensitivity. The preferred embodiment provides highsensitivity combined with high resolution in a compact geometry throughthe use of an RF ion guide loop system.

The preferred embodiment helps to solve the problem of the requirementfor physically long drift regions to achieve higher mobility resolutionand reduces the absolute voltage required to achieve the resolution,thus minimising the use of hazardous voltages and the risk of electricalbreakdown.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the present invention will now be described, byway of example only, and with reference to the accompanying drawings inwhich:

FIG. 1 shows an example of the voltages that must be applied to driftregions of different lengths in order to maintain the same electricfield strength along the different lengths of drift region;

FIG. 2 shows an ion mobility separator that translates an electric fieldregion along the drift length in order to separate ions;

FIG. 3 shows an ion mobility separator according to an embodiment of thepresent invention and having a circular drift length;

FIG. 4A shows a plan view of the arrangement of the electrodes in theembodiment of FIG. 3, and FIG. 4B shows a schematic of one of theelectrodes;

FIG. 5A shows a portion of the embodiment of FIG. 4A and FIGS. 5B to 5Dshow DC voltage potential profiles along this portion at different timesduring the ions separation process;

FIGS. 6A and 6B show an embodiment of a drift cell that is substantiallythe same as that shown in FIGS. 4A and 4B, except that the shape of theaperture in each electrode is different;

FIG. 7A shows a schematic of an ion entry/exit region of a drift cell ofa preferred embodiment, and FIGS. 7B and 7C show the potentials ofvarious parts of the entry/exit region at different times;

FIGS. 8A and 8B show different views of an ion mobility drift cellhaving an ion guide path that is of oval or racetrack geometry; and

FIG. 9A shows a plan view of the arrangement of the electrodes in anembodiment of the present invention, and FIG. 9B shows a schematic of across section through the drift cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The resolving power R of an ion mobility spectrometer (IMS) that uses auniform electric field is given by the expression:

$R = {\frac{t}{t_{FWHM}} = {\left( \frac{LEze}{16{kTln}\; 2} \right)^{0.5} = \left( \frac{Vze}{16{kTln}\; 2} \right)^{0.5}}}$

wherein t is the ion drift time through the drift region of the device;t_(FWHM) is the peak width at half height of the signal; L is the lengthof the drift region; E the electric field strength; z is the charge onthe ion being analysed; e is the unit electronic charge; V is thepotential difference across the drift region of the device (E=V/L); k isBoltzmann's constant; and T is the temperature of the drift gas in thedrift region.

It is apparent from the above expression that the potential difference Vacross the drift region of the IMS device can be increased in order toincrease the resolution of the device. However, increasing the potentialdifference across a fixed length of drift region will ultimately resultin electrical breakdown in the drift gas. In order to further increasethe resolution of the device it is therefore conventionally considerednecessary to increase the length of the drift region L. However, if thelength of the drift region L is increased then a greater potentialdifference must be applied across the drift region in order to maintainthe same electric field strength over the longer drift region.

FIG. 1 shows an example of the voltages that must be applied to driftregions of different lengths in order to maintain the same electricfield strength along the different lengths of drift region. If the driftregion only has a length L then a voltage V₀ may be applied at the exitof the drift region and a higher voltage V₁ may be applied at theentrance to the drift region in order to provide an electric fieldacross the drift region. The electric field drives ions through a driftgas that is present in the drift region, such that the ions separateaccording to their mobility through the drift gas as they pass throughthe drift region. If the length of the drift region is doubled to 2L andthe same voltage V₀ is applied at the exit of the drift region, then thevoltage applied at the entrance of the drift region must be increased toV₂ in order to maintain the same electric field strength along the driftregion of length 2L as was present along the drift region of length L.Similarly, if the length of the drift region is increased to 3L and thesame voltage V₀ is applied at the exit of the drift region, then thevoltage applied at the entrance of the drift region must be increased toV₃ in order to maintain the same electric field strength along the driftregion of length 3L as was present along the drift region of length L.This conventional approach ultimately leads to an extremely long driftregion and hence a large IMS device. Also, this conventional approachrequires the use of a relatively large potential difference in order toachieve the desired electric field strength along the relatively longdrift region. The use of high absolute voltages to achieve this can leadto electrical breakdown to the surroundings, which can be hazardous.

In order to avoid using such high voltages and long drift regions, adesired voltage gradient may be applied over only a portion of thelength of the drift region at any given time, such that the requiredelectric field is obtained in that portion of the drift region. Thevoltages that provide the electric field may then be progressed alongthe drift region such that the portion of the drift region in which theelectric field is applied keeps up with the drifting ions. This isillustrated in FIG. 2.

FIG. 2 shows a drift region of length L. An electric field is set upalong a portion of the drift region ‘dL’ by applying potentials V₀ andV₁ at spaced apart points of the drift region L. The electric fieldforces ions through the drift gas, causing them to separate according totheir ions mobilities as they pass through the drift gas. As the ionsprogress from the entrance towards the exit of the drift region L, theportion of the drift region dL over which the electric field is appliedis moved along the drift region L in a direction from the entrance tothe exit of the drift region. The portion of the drift region dL overwhich the electric field is applied is moved at a rate such that thedesired ions do not exit the electric field region dL as they passthrough the drift region L. This ensures that the ions of interestexperience a uniform electric field of the desired strength as they passalong the entire length of the drift region and without the need toprovide a large potential difference across the whole length of thedrift region L. Rather, as the electric field is only applied across aportion dL of the drift region L, a relatively small potentialdifference V₁-V₀ can be employed to achieve the desired electric fieldstrength. This technique therefore minimises the risk of electricalbreakdown that might otherwise be caused by the use of high absolutevoltages.

The resolution of a device of the kind described in relation to FIG. 2increases in proportion to the square root of the ratio L/dL, for afixed potential difference of V₁-V₀ over length dL. Therefore, in orderto increase the resolution of the device the total length of the driftregion L must be increased or the length of the electric field region dLmust be reduced. Increasing the total length of the drift region Lresults in an undesirably large IMS device. However, reducing the lengthof the electric field region dL may make it difficult to maintain theions of interest within the electric field region dL as they passthrough the drift region and spatially separate from each other.

FIG. 3. shows an IMS device 2 according to an embodiment of the presentinvention which may provide an improved resolution without suffering theabovementioned drawbacks. The IMS device 2 comprises a drift cell 4having electrodes for guiding ions along a drift length that is arrangedas a continuous circular geometry. Ions may be introduced into thedevice at an entry region 6. After the ions have entered the device 2they are caused to move around the drift length of the device byapplying voltages to the electrodes of the device. More specifically, apotential difference may be arranged or conveyed along the drift lengthso that the ions are urged along the drift length. A drift gas ispresent in the drift length and causes the ions to separate outaccording to their ion mobilities through the drift gas as they passalong the drift length. After the ions have performed their desiredseparation, they may be extracted from or allowed to exit the device 2at an exit region 6, which is preferably at the same location as theentry region 6. Ions are therefore preferably caused to perform at leastone complete cycle around the drift cell 4 before being extracted, i.e.a cycle from the entry/exit region 6 of the drift cell 4 all of the wayaround the drift cell 4 and back to the entry/exit region 6 of the driftcell. Ions may be caused to perform only a single cycle around the driftcell 4 or to perform multiple cycles around the drift cell 4 beforebeing extracted, depending upon the length over which the ions aredesired to be separated.

As mentioned above, a potential difference may be arranged or conveyedalong the drift length so as to cause the ions to cycle around the driftcell 4. If only a single cycle around the drift cell 4 is required thena fixed potential difference may be arranged across a fixed length ofthe drift cell 4 so as to drive the ions around the drift cell from theentrance region 6 to the exit region 6. Alternatively, a potentialdifference may be conveyed along the drift length so as to cause theions to cycle around the drift cell 4. In this embodiment a potentialdifference for driving ions through the device 2 may be arranged overonly a portion of the drift length at any given time. As the ions travelaround the drift cell 4, the length of the drift region over which thepotential difference is applied is conveyed around the drift cell 4 sothat the desired ions are always maintained in a region across which thepotential difference is applied. If the ions are only desired to travelone cycle around the drift cell 4, then the drift length across whichthe potential difference is applied may be caused to travel around thedrift cell 4 only once. However, if the ions are desired to travelmultiple cycles around the drift cell 4, then the drift length acrosswhich the potential difference is applied may be caused to travel aroundthe drift cell 4 multiple times along with the ions. In modes whereinthe ions cycle around the drift cell 4 multiple times, the ion entry andexit region 6 may be deactivated so that the ions pass the entry andexit region 6 unimpeded, until it is desired to extract the ions.

FIG. 4A shows a preferred embodiment of the arrangement of theelectrodes 8 in the drift cell 4 of FIG. 3 from a plan view. The driftcell 4 may be formed from a plurality of apertured electrodes 8 that arearranged in a circle and such that each electrode 8 lies in a plane thatextends radially outward from the centre of the drift cell 4. An exampleof an apertured electrode 8 having a circular aperture 10 is shown inFIG. 4B. Voltages are applied to the electrodes 8 so as guide ionsthrough the apertures 10 in the successive electrodes 8 and hence aroundthe drift cell 4. More specifically, RF voltages may be applied to theelectrodes 8 so as to radially confine the ions and provide an ionguiding path through the apertures 10 of the electrodes 8. Alternateelectrodes 8 in the drift cell 4 are preferably applied with differentphases of an RF voltage source. Alternate electrodes 8 in the drift cell4 are preferably applied with opposite phases of the RF voltage source,i.e. when a given electrode 8 is at an RF phase of 0 degrees theadjacent electrodes 8 are preferably at 180 degrees. A DC voltagegradient is applied to at least some of the electrodes 8 and ispreferably superimposed on the RF voltages so as to provide an axialelectric field that urges ions to drift through the drift gas and aroundthe drift cell 4.

In the example shown in FIG. 4A it can be seen that one electrode 8 a ismaintained at a relatively high voltage V₁ and an adjacent electrode 8 bis maintained at a relatively low voltage V₀. This causes ions to beforced away from the electrode 8 a at high voltage V₁ and to pass aroundthe drift cell 4 in an anti-clockwise manner towards the electrode 8 bat low voltage V₀. At least some of the electrodes 8 that are arrangedbetween the two electrodes 8 a,8 b held at V₁ and V₀ also preferablyhave DC potentials applied to them so as to maintain a voltage gradientthat decreases between said two electrodes 8 a,8 b. For example,progressively smaller DC voltages may be applied to the electrodes 8around the drift cell 4 so as to create a voltage gradient that drivesthe ions along the drift length. The DC voltage gradient may begenerated using a resistor chain 12 coupled to the electrodes 8 formingthe drift cell 4 and across which a potential difference is applied. Itwill be appreciated that although a decreasing voltage gradient has beendescribed for urging positive ions around the device, an increasingvoltage gradient may be used to urge negative ions around the device.

In the above example, the DC potential difference applied to the deviceis arranged along a fixed length of the device. Alternatively, a DCpotential difference may be arranged along only a portion of the lengthof the drift region so as to form an axial electric field region andthis axial electric field region may then be moved around the devicewith the ions. In the latter embodiment, DC voltages may be applied toonly some of the electrodes 8 forming the drift cell 4 so as to form aDC voltage gradient and axial electric field region along only a portionof the drift cell 4. The electrodes 8 to which these DC voltages areapplied may then be changed with time so that length over which theaxial electric field is maintained is moved around the drift cell,preferably in a manner such that as the ions pass around the drift cellthey always remain within the electric field region. This ensures thatthe ions experience a uniform electric field strength as they passaround the drift cell 4. The axial DC voltage gradient may beprogressively stepped around the device in steps of single electrodes 8or in steps of multiple electrodes 8. However, it is observed thatincreasing the number of electrodes 8 by which the voltage gradient isstepped around the device effectively reduces the range of ionmobilities which can be retained in the axial electric field region.

FIG. 5A shows the portion of the drift cell 4 of FIG. 4A at which ionsmay enter or exit the drift cell. FIGS. 5B to 5D show DC potentialprofiles along this portion of the drift cell 4 at different timesduring the ions separation process. In order to facilitate ion entry andion exit at the same point on the circumference of the drift cell 4,ions are preferably arranged to enter or leave the drift cell in aregion 6 of low DC potential. As shown in FIG. 5B, it is preferred thatsubstantially no DC voltage gradient is arranged along the entrance/exitregion 6 at the time that ions enter the drift cell 4. Once the ions arewithin the drift cell 4, an axial DC potential difference is thenpreferably applied. DC potentials are applied to the electrodes 8 aroundthe drift cell 4 so as to form a voltage gradient as shown in FIG. 5C. Arelatively high voltage V₁ may be applied at a first electrode 8 a andprogressively smaller DC voltages may be applied to the electrodes 8around the drift cell 4 up to the last electrode 8 b which is at V₀. Ascan be seen from FIG. 5C, the potential at the entrance region 6 isrelatively high and the ions experience an axial electric field andbegin travelling along the potential difference and around the device ina clockwise manner.

As described above, it is desired to apply a relatively high DCpotential at the ion entrance region 6 in order to cause ions to beginto drift around the drift cell 4. It is also desired that the ions exitthe device at a region 6 in which the DC potential is low orsubstantially zero. However, it is preferred for the ions to enter andexit the drift cell 4 at substantially the same location 6. According tothe preferred embodiment both of these functions are enabled, with ionsentering and exiting the device at the same location 6. The preferredembodiment achieves this by changing the DC potentials applied to theelectrodes 8 with time such that the position of the voltage gradientmoves around the drift cell 4. As described above, the potential profileshown in FIG. 5C causes ions to begin to drift around the drift cell 4after they have entered the device. As the ions move around the driftcell the location of the potential difference also moves around thedrift cell in a manner such that the ions experience the same uniformvoltage gradient. By the time that the ions have passed around the driftcell to the exit region 6, the voltage gradient has rotated around thedrift cell to the position shown in FIG. 5D. It can be seen that the lowDC voltage V₀ has moved around the drift cell 4 to the position atelectrode 8 d and the high DC voltage V₁ has moved around the drift cell4 to the position of electrode 8 c. The portion of the axial DCpotential difference that is arranged at the exit region 6 of the driftcell 4 therefore has a relatively low DC potential, enabling ions toexit the drift cell 4 in a region of substantially no or low DCpotential.

As described above, if only a single cycle of the ions around the driftcell 4 is required, then the DC potentials preferably progress aroundthe circumference such that the high voltage region at the entry/exitregion 6 (substantially at V₁) is replaced by the low voltage region(substantially at V₀) by the time that the ions reach the exit region 6.In an alternative method, ions may enter the drift cell 4 and may thenbe transported to a confining region (not shown) that is located at somepoint away from the entry/exit region 6. The ions may be transported tothe confining region using electric fields, such as by applying DCvoltages to the electrodes 8. After the ions have been moved to theconfining region a relatively high DC voltage V₁ may be applied at theconfining region and a relatively low DC voltage V₀ may be applied atthe entry/exit region 6, causing the ions to move around the drift cell4. In this manner, it is not required to change the location of theaxial DC voltage gradient that separates the ions in order to be able toextract ions at an exit region 6 having substantially no DC voltage.This approach of moving the ions away from the entry region prior tomobility separation can also be used in modes wherein ions are caused tocycle around the drift cell 4 multiple times.

FIGS. 6A and 6B show an embodiment that is substantially the same asthat shown in FIGS. 4A and 4B, except that the shape of the aperture 10in each electrode 8 is different. FIG. 6B shows a perspective view ofsome of the electrodes 8 making up the drift cell 4 of FIG. 6A. It willbe seen that each electrode 8 has a slotted aperture 10 through whichthe ions pass as they travel around the drift cell 4. Each slot 10preferably has its width arranged in the direction of the radius R ofthe cell drift 4 and its length perpendicular to the radius R of thedrift cell 4. The use of slots 10 as opposed to other shaped apertures10 allows a relatively high charge capacity of ions to be contained bythe electrodes 8 whilst confining the ions at substantially the sameradial distance R from the centre of the drift cell 4. This minimisesthe differences in electric field and path length that ions experienceas they pass around the drift cell 4. It will be appreciated that ifrelatively large circular apertures 10 were used to confine a highcharge density then ions at a relatively large radial distance R fromthe center of the drift cell 4 would experience a significantlydifferent path length to ions at a relatively small distance R from thecentre of the drift cell 4. It can be seen that the plane in which eachelectrode 8 is arranged is preferably aligned in a radial direction fromthe centre of the drift cell 4. The electrodes 8 therefore diverge fromeach other with increasing distance from the centre of the drift cell 4,causing the RF radial confinement to become less effective for ions atincreasing radial distance R from the centre of the drift cell 4. Theuse of slotted apertures 10 enables a relatively high number of ions tobe located at substantially the same radial position with respect to thecentre of the drift cell 4, such that the ions experience substantiallythe same RF containment field.

In a closed loop device such as any of those described above, the rangeof mobilities that can be analysed in a given experiment is determinedby the physical length of the drift field and the temporal length of themobility experiment. For example, if the DC voltage gradient progressesaround the drift cell 4 at a rate such that the lowest mobility ionspecies is retained at the point of the applied high potential V₁, thenafter a given time the highest mobility species will reach the end ofthe DC voltage gradient where the low potential V₀ is applied and nofurther ion mobility separation will occur for this species. If the ionsof highest mobility have not reached the ion extraction point 6 by thistime then there is a possibility that ions of lower mobility willre-merge with the ion of higher mobility, thus losing the ion mobilityseparation. This situation may be exacerbated if higher mobilityresolution is required since more cycles of the ions around the driftcell 4 are required and so the temporal length of the mobilityseparation increases.

The present invention contemplates providing relatively high resolutionion mobility separation on a selected range of ion mobility species bysynchronising the rate at which the DC voltage gradient is cycled aroundthe drift cell 4 with the rate at which the ion mobility range ofinterest cycles around the drift cell 4. The synchronisation may beperformed so as to allow ion species with undesirably low mobility to‘fall off’ the DC voltage gradient at the high potential V₁ end andallow ions with undesirably high ion mobilities to leave the DC voltagegradient at the low potential V₀ end and re-merge with the other ions.The cycling rate of the DC voltage gradient can then be selected so thatthe low potential V₀ is arranged at the ion exit region 6 when thedesired range of ion mobilities reaches this region for extraction. Itis also contemplated that undesired ions that reach one of both ends ofthe DC voltage gradient may be eliminated or discarded from the system.This may be achieved, for example, by removing the RF ion confinementvoltages applied to the electrodes 8 at points outside of the lengthalong which the DC voltage gradient is maintained. The ions are then notradially confined at these points and will be lost to the system throughdiffusion.

FIG. 7A shows a schematic of an ion entry/exit region 6 of the driftcell 4. FIGS. 7B and 7C show the potentials of various parts of theentry/exit region 6 at different times. Referring to FIG. 7A, theentry/exit region 6 comprises an array of entry/exit electrodes 14arranged between two adjacent ones of the apertured electrodes 8 a,8 b.The entry/exit electrodes 14 preferably comprises a plurality of rows ofupper electrodes and a plurality of rows of lower electrodes, whereinthe rows are aligned with the planes in which the apertured electrodes 8a,8 b are located. Each row of entry/exit electrodes 14 is preferablymade up of a plurality of electrodes that may be axially separated alongthe length of each row. In order to radially confine the ions in theentry/exit region 6, RF potentials are preferably applied to theentry/exit electrodes 14. The same phase of the RF voltage supply ispreferably applied to all of the entry/exit electrodes 14 that are inthe same row. Different phases, preferably opposite phases, of the RFvoltage supply are preferably applied to adjacent rows of the entry/exitelectrodes 14 in order to radially confine the ions.

During ion entry into the drift cell 4, voltages may be applied to theentry/exit electrodes 14 so as to generate a small electric field in they-direction that encourages ions into the drift cell 4 through the sideof the drift cell 4. Voltages may also be applied to the entry/exitelectrodes 14 at the opposite side of the entry/exit region 6 to whichthe ions enter, so as to prevent the ions passing straight through andout of the drift cell 4. When it is desired to begin ion mobilityseparation, voltages applied to the entry/exit electrodes 14 areselected so as to apply an axial electric field in the x-direction thatcauses the ions to pass around the drift cell 4. FIG. 7B shows thepotential profile formed by the entry/exit electrodes 14 and theapertured electrodes 8 a,8 b on either side thereof at this point intime. Potential profile 16 is the profile due to apertured electrode 8a, potential profile 18 is the profile due to apertured electrode 8 b,and potential profile 20 is the profile due to entry/exit electrodes 14.When it is desired to cause ions to exit the entry/exit region 6, thepotentials applied to the entry/exit electrodes 14 are preferablychanged such that an electric field urges ions out of the device iny-direction. FIG. 7C shows the potential profile formed by theentry/exit electrodes 14 and the apertured electrodes 8 a, 8 b on eitherside thereof at this point in time. Potential profile 16 is the profiledue to apertured electrode 8 a, potential profile 18 is the profile dueto apertured electrode 8 b, and potential profile 20 is the profile dueto entry/exit electrodes 14. Alternatively, it is contemplated that avoltage is successively applied to the entry/exit electrodes 14 in they-direction towards the exit of the device so that a travellingpotential wave propels ions out of the exit.

It will be appreciated that drift cells 4 having continuous ion guidingpaths of shapes other than circular paths are also contemplated as beingwithin the scope of the present invention. For example, a continuousoval or racetrack ion guide geometry may be employed.

FIG. 8A shows a plan view of an ion mobility drift cell 4 having an ionguide path that is of oval or racetrack geometry. The drift cell 4 maybe operated and constructed in substantially the same manner as any oneof the embodiments described above, except that the electrodes 8 arearranged to form an ion guiding path that extends in a oval or racetrackshape, rather than a circular shape. The electrodes 8 of the drift cell4 may be electrically connected to a printed circuit board. Ions enterand exit the drift cell 4 at ion entry/exit region 6. This entry/exitregion 6 may be constructed in the same manner as in the aboveembodiments. Alternatively, the entry/exit region 6 may be configured sothat ions can enter and exit the drift cell 4 in a direction thatextends upwards above or downwards below the drift cell 4, rather thanthrough the side of the drift cell 4 as in the above-describedembodiments.

FIG. 8B shows the embodiment of FIG. 8A from a side view. The drift cell4 is arranged inside a chamber 22 that is filled with drift gas. Ionsare guided into and out of the chamber 22 using RF ion guides 24,26. TheRF ion guides 24,26 are also coupled with the ion entry/exit region 6 ofthe drift cell 4 such that ions can be guided into the drift cell 4 andout of the drift cell 4. In this embodiment, ions are guided into thechamber 22 and into the entry/exit region 6 of the drift cell 4 from adirection below the drift cell 4 by input ion guides 24. If the ions aredesired to be separated by their ion mobility then the ions are urgedaround the oval or racetrack ion path of the drift cell 4, in the samemanner as in the above described embodiments. As the ions pass along theion path they separate according to their ion mobility through the driftgas that is present in the chamber 22 and hence the drift cell 4. Whenions are desired to be extracted from the drift cell 4 they are ejectedin a direction upwards above the drift cell 4 and into the ion guides26. The ions are then guided out of the chamber 22 by the ion guide 26.On the other hand, if ion mobility separation of the ions is notrequired then ion species can be caused to pass from the input ionguides 24 to the output ion guides 26 directly through the entry/exitregion 6 of the drift cell 4 and without passing around the drift cell4. In other words, the drift cell 4 may be operated in a by-pass mode.

In a preferred mode of operation, it is possible to extract ions havinga desired range of ions mobilities from the drift cell 4. This isachieved by causing ions to traverse around the drift cell 4 so thatthey separate and then synchronising the activation of an ejection pulseat the ion entry/exit region 6 with the time at which the ions ofinterest are at the entry/exit region 6. The desired ions are thereforeejected from the drift cell 4 and the other ion species remaining in thedrift cell 4 can continue to pass through the drift cell 4 and separateaccording to ion mobility. Alternatively, the remaining ions may bediscarded from the drift cell 4, for example, by removal of the RFvoltages from the electrodes 8 such that the ions are no longer radiallyconfined within the drift cell.

The ejected ions having the desired ion mobilities can be immediatelytransported away from the drift cell 4 to a mass analyser or detector.Alternatively, such ions may be trapped in a storage region whilst thenext mobility cycle occurs in the drift cell 4 and until more ions ofthe same ion mobility range are ejected from the drift cell 4 into thestorage region. After sufficient mobility cycles have been performed toaccumulate the desired number of ions in the storage region, these ionsmay then be transported to an analyser for further analysis or to adetector. This method may be used to increase the ion signal of thedesired ions. Additionally, or alternatively, the desired ions that havebeen ejected from the drift cell 4 may be caused to fragment ordissociate and then reintroduced back into the drift cell 4 such thatthe ion mobilities of the fragment or daughter ions can be analysed bythe drift cell 4.

It will be appreciated that in a mode of operation the ion guiding pathof the drift cell 4 may be used to store ions and that the drift cell 4may operate as an ion storage device.

Although the electrodes forming the drift cell have been describedhereinabove as being apertured electrodes, it is also contemplated thatother geometries of electrodes may be used to guide ions around thedrift cell.

FIG. 9A shows a preferred embodiment of the arrangement of theelectrodes in the drift cell 4 from a plan view. Rather than the driftcell 4 being formed from a plurality of apertured electrodes 8 that arearranged in a circle, the drift cell is divided into segments 28,wherein each segment 28 comprises a plurality of layers of electrodes,as shown in FIG. 9B.

FIG. 9B shows a cross-section through one of the segments 28 in FIG. 9A.Each segment 28 is formed from a top electrode 30, a bottom electrode 32and a plurality of layers of electrodes 34 arranged therebetween. Eachlayer comprises two laterally spaced apart electrodes 34 arranged suchthat these electrodes 34 form side electrodes. The side electrodes 35are stacked in columns so as to define a space 36 between the columns ofside electrodes 34, and between the top and bottom electrodes 30,32. Thetop, bottom and side electrodes 30,32,34 are substantially planar andextend around the longitudinal direction of the drift cell 4 so as toform a segment 28 of the drift cell 4 as shown in FIG. 9A. The planarelectrodes 30,32,34 extend in the plane that ions travel in, in use.

Ions are radially confined within the space 36 between the sideelectrodes 34, top electrode 30 and bottom electrode 32. In order toachieve this confinement, RF potentials are applied to the sideelectrodes 34. The same phase of an RF voltage source is preferablyapplied to the two side electrodes 34 in each layer. Different phases ofthe RF voltage source are preferably applied to the side electrodes 34in adjacent layers. The side electrodes 34 in any given layer arepreferably supplied with an opposite RF voltage phase to the sideelectrodes 34 in the adjacent layers. By applying RF potentials to theside electrodes 34, the ions are laterally confined within the space 36between the side electrodes 34. RF potentials may also be applied to thetop and bottom electrodes 30,32 so as to confine ions within the space36 in the vertical direction. However, it is preferred that only DCpotentials are applied to the top and bottom electrodes 30,32 so as toconfine the ions in the vertical direction.

Although the drift cell 4 has a different electrode configuration to theearlier described embodiments having apertured electrodes 8, theoperation of the drift cell 4 is substantially the same. A DC voltagegradient is applied to at least some of the electrodes so as to providean axial electric field that urges ions to drift through the drift gasand around the drift cell 4. The DC voltage gradient may be formed bysupplying different DC voltages to the electrodes of different segments28 of the drift cell 4. Different DC voltages may be supplied to the top30 and/or bottom 32 electrode in different segments 28 so as to form thevoltage gradient. Additionally, or alternatively, different DC voltagesmay be supplied to the side electrodes 34 of different segments 28 so asto form the voltage gradient. For example, progressively smaller DCvoltages may be applied to the electrodes of the different segments 28around the drift cell 4 so as to create a voltage gradient that drivesthe ions along the drift length.

As described in relation to the earlier embodiments, the DC potentialdifference applied to the device may be arranged along a fixed length ofthe device. Alternatively, the DC potential difference may be arrangedalong only a portion of the length of the drift region so as to form anaxial electric field region and this axial electric field region maythen be moved around the device with the ions. The exit region 6 may beconfigured as described previously.

Although the present invention has been described with reference topreferred embodiments, it will be understood by those skilled in the artthat various changes in form and detail may be made without departingfrom the scope of the invention as set forth in the accompanying claims.

1. A method of separating ions according to their ion mobilitycomprising: providing an RF ion guide having a plurality of electrodesarranged to form an ion guiding path that extends in a closed loopsupplying DC voltages to at least some of said electrodes, wherein saidDC voltages urge ions to separate according to their ion mobility as theions pass along the ion guide.
 2. The method of claim 1, wherein the ionguide comprises an ion entry/exit region configured for introducing ionsinto the ion guide in one mode and for extracting ions from the ionguide in another mode, wherein the ion entry/exit region is at a fixedlocation on the ion guide.
 3. The method of claim 1, wherein theelectrodes of the ion guide are axially spaced along a longitudinal axisof the ion guide and wherein the DC voltages are applied to differentones of said axially spaced electrodes so as to form a DC voltagegradient. 4-5. (canceled)
 6. The method of claim 1, wherein asubstantially uniform DC voltage gradient is arranged along a DC voltagegradient region.
 7. The method of claim 1, wherein the ions separate outaccording to their ion mobility within a DC voltage gradient region.8-11. (canceled)
 12. The method of claim 1, wherein RF voltages areapplied to the electrodes so as to confine ions within the ion guidingpath along a DC voltage gradient region, and RF voltages are not appliedto at least some of the electrodes at one or more regions of the ionguide outside of the DC voltage gradient region such that ions are notconfined within said one or more regions of the ion guide and are lostfrom the ion guide at these regions.
 13. An ion mobility separatorcomprising: an RF ion guide having a plurality of electrodes arranged toform an ion guiding path that extends in a closed loop; and a DC voltagesupply arranged and adapted to supply DC voltages to said electrodes,wherein in use said DC voltages urge ions to separate according to theirion mobility as they pass along the ion guide.
 14. The method of claim1, wherein as time progresses a portion of the ion guide along which aDC voltage gradient is maintained is moved along the ion guide.
 15. Themethod of claim 14, wherein the DC voltage gradient moves around the ionguide at a rate such that at least some of said ions continually remainwithin a DC voltage gradient region as they travel around the ion guideand until such ions are extracted from the ion guide at an exit regionof the ion guide.
 16. The method of claim 15, wherein the ions remainwithin the DC voltage gradient as they travel only a single cycle aroundthe closed loop ion guide, or wherein the ions remain within the DCvoltage gradient as they repeatedly travel multiple cycles around theclosed loop ion guide.
 17. The method of claim 14, wherein the DCvoltage gradient moves around the ion guide at a rate such thatundesired ions having an ion mobility above a first threshold valueand/or below a second threshold value do not continuously remain withina voltage gradient region as the region is moved around the ion guide.18. The method of claim 17, wherein undesired ions having an ionmobility above the first threshold value exit the low potential end ofthe voltage gradient region and/or undesired ions having an ion mobilitybelow the second threshold value exit the high potential end of thevoltage gradient region.
 19. The method of claim 17, wherein theundesired ions that do not continuously remain within the DC voltagegradient region are not extracted from the ion guide at an exit regionof the ion guide.
 20. The method of claim 14, wherein the rate at whichthe DC voltage gradient moves around the ion guide is synchronised withthe rate at which ions of interest are urged around the ion guide by thevoltage gradient such that the ions of interest reach an exit region ofthe ion guide at a time when the minimum potential of the voltagegradient is arranged at the exit region of the ion guide.
 21. The methodof claim 1, wherein DC voltages are only applied to some of theelectrodes of the ion guide such that a DC voltage gradient is arrangedalong only a portion of the length of the ion guide at any given time.22. The method of claim 1, wherein a DC voltage gradient is arrangedover substantially the whole length of the ion guiding region at anygiven time.
 23. The method of claim 1, wherein: the electrodes areapertured electrodes that are aligned such that the ions are guidedthrough the apertures of the electrodes as they travel along the ionguiding path; the apertures in the electrodes are slotted apertures; andthe electrodes are arranged such that at least a portion of the ionguiding path is curved and so has a radius of curvature, wherein eachslot has its minimum dimension substantially parallel with said radiusand its maximum dimension substantially perpendicular to said radius.24. The method of claim 1, wherein the electrodes are arranged such thatthe closed loop ion guiding path is substantially circular or oval. 25.An ion mobility separator comprising: an RF ion guide having a pluralityof electrodes arranged to form an ion guiding path that extends in aclosed loop; and a DC voltage supply arranged and adapted to supply DCvoltages to said electrodes, wherein in use said DC voltages move aroundthe ion guide at a rate synchronised with a rate at which ions ofinterest are urged around the ion guide by said DC voltages.
 26. A massspectrometer comprising an ion mobility spectrometer as claimed in claim13.